1. Field of the Invention
The present invention relates to a control system for controlling an internal combustion engine which discharge exhaust gases through an emission purifier.
2. Description of the Prior Art
Internal combustion engines equipped with a three-way catalytic converter generally fail to provide a desired level of emission purifying performance unless the catalytic converter is activated with its temperature raised to a certain level. Therefore, it has heretofore been an important task to achieve a desired level of emission purifying performance immediately after the internal combustion engine has started its operation when the temperature of the catalytic converter is relatively low.
One known attempt to carry out such a task is disclosed in Japanese laid-open patent publication No. 6-101456, for example.
According to the disclosed arrangement, the ignition timing of an internal combustion engine is retarded with respect to normal timing for thereby increasing the temperature of exhaust gases, so that the temperature of a catalytic converter combined with the internal combustion engine will quickly be increased for enabling the catalytic converter to provide a desired level of emission purifying performance at an early stage. Furthermore, it is also disclosed to increase the quantity of intake air drawn into the internal combustion engine for preventing the torque and rotational speed thereof from being lowered due to the retarded ignition timing.
Specifically, a corrective quantity for retarding the ignition timing while the internal combustion engine is idling is determined, using a data table or the like, from operating conditions including a coolant temperature at the time of starting the internal combustion engine, and the quantity of intake air to be increased is determined depending on the determined corrective quantity. The ignition timing and the quantity of intake air are controlled in a feedforward fashion according to the determined values therefor.
According to the disclosed arrangement, however, the process of determining the corrective quantity for retarding the ignition timing depending on various coolant temperatures and the control timing for controlling the ignition timing, and the process of determining the quantity of intake air depending on the correction of the ignition timing and the control timing for controlling the quantity of intake air have to be preset in a closely correlated manner. Therefore, a control system for carrying out the above control processes is complex, and settings for enabling the control system to achieve a desired level of emission purifying performance tend to become inappropriate. As a result, it is difficult to stably control the ignition timing and the quantity of intake air for activating the emission purifier at an early stage for a desired level of emission purifying performance, without changing the torque and rotational speed of the internal combustion engine.
Since the quantity of intake air is determined depending on the corrective quantity for ignition timing according to the invention disclosed in the above publication, a delay in controlling the quantity of intake air with respect to controlling the ignition timing has to be taken into account for the overall control process, resulting in control system complexities and setting difficulties.
Internal combustion engines generally are subject to less friction in various parts and higher rotational speed as they are warmed up. The manner in which the various parts suffer less friction is affected by various elements including the nature of a fuel used, the type, amount, and temperature of a lubricant used, and the temperature of a coolant used. For this reason, there are instances where the friction of various parts drops faster than normal and instances where friction of various parts drops earlier than normal.
According to the disclosed arrangement of the above publication, as described above, a corrective quantity for retarding the ignition timing while the internal combustion engine is idling is determined, using a data table or the like, from operating conditions including a coolant temperature at the time of starting the internal combustion engine, and the quantity of intake air to be increased is uniformly determined depending on the determined corrective quantity. Therefore, it is difficult to adequately suppress variations in the rotational speed due to the reduction in the friction of various parts of the internal combustion engine owing to various elements as referred to above.